diagnose example code fix

[jussipo]

line 67 of pn53x-diagnose.c:
const uint8_t pncmd_diagnose_communication_line_test[] = { Diagnose, 0x00, 0x06, 'l', 'i', 'b', 'n', 'f', 'c' };
should be (if just testing reply or communication is ok):
const uint8_t pncmd_diagnose_communication_line_test[] = { Diagnose, 0x00, 0x00, 'l', 'i', 'b', 'n', 'f', 'c' };

pn532 datasheet informs NumTst code 0x00 is reply test command , 0x06 is card presence test cmd.
Example code checks if string libnfc is echoed back...

[jussipo]

oops, discard this.